The Mod Bob antenna on 160 – 30M ends up looking like a single vertical on 160 and 80M and on 30M it models just like a Bobtail Curtain. It does look like a dipole on 40M, but my 2 element 40M phased array almost always beats it, so the Mod Bob is used on 160, where it is resonant, 80M, where it is a poor match of 20:1 and 30M where it is about 6:1. My guess (and I haven’t tried this) is that a 6:1 balun would work on 30M – but then mess up 160M, and that 80M will always be a tough match.
I read this article by antenna guru Richard Measures, AG6K:
And decided to try the Palstar BT1500A, which seems to be designed exactly with AG6K in mind. That link is an updated version of the February 1990 QST article.
You’ll see that Palstar did a very nice job staying faithful to that design, and since I had never tried such an antenna tuner, this time it just seemed to fit. You’ll see in an earlier blog post that the BT1500A worked out in a test exactly as I had hoped – that one switch combination would result in just having to tune the L and C constrols – because then it would make it much easier to build a stepper motor controlled remote tuner with less $100 in “maker” style parts. In fact, with just a rotator control line, I can supply the Arduino and stepper motors with 5V and the BT1500A with the 12V it needs. In fact, here’s a way to do that:
And here is are the specs of a few options – both are less than $20:
There’s enough power for all components in the remote box, and with the 8 wire heavy duty rotator control wire, it should be just fine. I could even put the power supply just a few feet away from the antenna feed in the basement, but that’s not really necessary.
VU2ESE also has the same idea that I do – but he is using low power:
With 100 watts or less you can get away with a lot and never know how efficient and antenna system is. K9YC likes to talk about “commiting unnatural acts” when trying to use a Hi Z ladder line fed doublet as a multi band antenna. He says that the ladder line – because it can’t be choked can pick up noise and also become part of the radiator. In my case – its hard to say if this is a good or bad thing.
More than anything, the Mod Bob – being resonant on 160M is esily what matters. In fact, if 80 and 30M don’t work out or cause 160M to be less efficient using this feed scheme – I’d drop the BT1500A at the antenna feed point. But I have to try this.
The next test though will have to be with high power on 160, 80 and 30M and see if there are any issues at 1500 watts. I will have to set the SPE Expert 1.3K FA to bypass its own internal tuner for these bands. I will still use the internal tuner on my 40 and 20M resonant antennas.
W8JI has an opinion and has looked into the balanced vs. unbalanced tuner and baluns at the input and output of tuners:
Tom argues that maybe my old Palstar AT2K in the shack with a 1:1 current choke at the antenna might not be any worse than putting the BT1500A at the antenna.
This weekend – with full power and the tuner mounted at the antenna I will find out. I expect that maybe Tom, W8JI is right – because when I had the Palstar AT2K in the shack with the ACOM 1500, it never arc’ed or caused any issues.
The coax feed from the shack to the Mod Bob does go through the Array Solutions Rat Pack remote switch, so maybe moving the tuner out to the antenna will be better in that regard – I’m really not sure. The length is only about 60′, so I doubt that losses are a big deal with the tuner in the shack or at the antenna. Not really sure though. Not sure exactly what happens along that feedline – as I sure hope the heavy duty 1:1 current choke does its thing – which I am confident that it does.
I look forward to my high power test this weekend – the big test will be to see if there are any issues on the three bands with high power. The only downside with the SPE amplifier is that it is much more “finicky” than the ACOM 1500 tube amp in respect to SWR. But this might be a really good thing – because it is forcing me to test and try this different approach.
I do know that coax fed dipoles on every band is a great way to go – but on the low bands this is simply not possible. I also know that my radial field for an all band (or all low band) vertical ends up being hatted and causes serious voltage issues on components at the base of that antenna – I have fried a switch that I used to switch in different L and C components in an unbalanced scheme.
I am avoiding the loss of my radial field with the Mod Bob, and sure hope that I have no issues on 160, 80 and 30M with this weekends test.
I hope it proves that this was a good way to go – we shall see.